1. Field of the Invention
The present invention relates to a flat panel display device having an array of cold cathodes each served as an electron source.
2. Description of the Related Art
In recent days, the study and development of a flat panel display device is being active. As examples of such a display device, a liquid crystal display (LCD), an electroluminescent display (ELD) and a light-emitting diode display (LED) are now made commercially available. Those displays are, however, inferior to a cathode-ray tube (CRT) with respect to luminance of emitted light, resolution and color. Hence, several kinds of thin CRTs have been developed. Those thin CRTs are largely grouped into ones having dot electron sources like the normal CRT, ones having a plurality of linear cathodes, and ones having cathodes extending on the overall fluorescent surface. Concretely, an electron multiplication type CRT, a horizontal address and vertical electrostatic deflection type CRT, a MSD (Matsushita Denki) type CRT, and a flat CRT are made commercially available. In particular, the MDS type CRT is made thinner than the other CRTs so that the 10-inch CRT have a depth of 9.9 cm.
All the CRTs described above contain a fluorescent material coated on their display surface as a luminophore material. To enhance the luminance, in general, there is coated an aluminum (Al) thin film having a high light-reflecting factor and electron permeability on the opposite side to the side of the fluorescent layer viewed from the user. The coat of this film is referred to as an aluminizing or metal-backing method.
However, even such a high-performance thin CRT becomes critical in coping with the current increase of used information or the social phenomenon where each person may have his or her portable TV. The demands for lighter, thinner and shorter image display device have rapidly risen. In particular, a great remark is placed on an image display device where field-emission type cold-cathode microguns are disposed in a matrix. As this type of image display device, "Microtips Fluorescent-Display" by R. Heyer, et. al. has been published in the Japan Display 1986 Conference. The microgun is made of a molybdenum cold-cathode tip. An electric field takes place between the cold-cathode tip and a gate electrode located adjacent to the top of the tip, so that electrons are emitted through the effect of the electric field. The distance between an anode electrode surface made of a luminophore material and the gate electrode is about 100 .mu.m. It means that the manufacture of a super thin and high-definition image device is made possible if it is used. This device can be used for a large flat display TV or a display provided in a portable electronic equipment.
To progress the study and development of the application of a flat panel display device having electric-field emission type cold-cathode microguns into the portable electronic equipment, it is necessary to keep its operating voltage as low as possible. The reduction of the voltage can reduce the size of battery and this reduction of the size results in making it for the electronic equipment to be more portable. To lower the operating voltage, it is necessary to lower a threshold voltage between the cathode electrode and the gate electrode, the threshold voltage being a critical voltage for emitting electrons through the effect of an electric field. Currently, the threshold voltage is about 50 V and the smaller operating voltage is 80 V between the cathode electrode and the gate electrode and about 400 V between the cathode electrode and the anode electrode. The operating voltage is now being acceleratingly ameliorated. It will be several tens V some years later.
However, the energy of electrons colliding with the fluorescent material coated on the anode electrode surface and the luminance of the fluorescent material are made lower as the operating voltage is getting lower. This results in lowering the luminance of the displayed image, thereby making the image quality worse.
To overcome this shortcoming, it is possible to divert the metal-backing structure which has been used in the thin type thin CRTs mentioned above. In a case that a metal back structure (Al film) is used in the flat panel display device having microguns of electric field type cold-cathodes, the emitted electrons pass through the Al film into the fluorescent layer surface, because the cathodes are located on the opposite side to the display surface. Some of the emitted electrons are absorbed in the Al film and the remaining of the electrons reach the fluorescent layer. In general, the Al film is made as thick as about 0.2 .mu.m by considering prevention of ion penetration and oxidation in the working process. If the Al film having a thickness of about 0.2 .mu.m is used, to enhance a penetration factor of electron energy to 50% or higher, the electrons need to have energy of about 7 KeV or more. If the Al film thickness is about 0.05 .mu.m, the electrons need to have energy of about 3.5 Kev or more. The current CRT device may supply such high energy to the electrons. However, the flat panel display device to be applied to the portable electronic equipment disables to supply so high energy.